troubleshooting dns
TRANSCRIPT
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IP Addressing: DNS Configuration Guide,Cisco IOS Release 12.4T
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CON T EN T S
Configuring DNS 1
Finding Feature Information 1
Prerequisites for Configuring DNS 1
Information About DNS 1
DNS Overview 1
Hostnames for Network Devices 2
Domains Names for Groups of Networks 2
Name Servers 2
Cache 2
Name Resolvers 2
Zones 3
Authoritative Name Servers 3
DNS Operation 3
How to Configure DNS 3
Mapping Hostnames to IP Addresses 4
Customizing DNS 5
Configuring DNS Spoofing 7
Configuring the Router as a DNS Server 8
Examples 10
Debugging Output for Relaying a DNS Query to Another Name Server Example 11
Debugging Output for Servicing a DNS Query from the Local Host Table Example 11
Disabling DNS Queries for ISO CLNS Addresses 11
Verifying DNS 12
Configuration Examples for DNS 13
IP Addresses Example 13
Mapping Hostnames to IP Addresses Example 13
Customizing DNS Example 13
Configuring DNS Spoofing Example 14
Additional References 14
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Feature Information for DNS 15
Dynamic DNS Support for Cisco IOS Software 17
Finding Feature Information 17
Restrictions for Dynamic DNS Support for Cisco IOS Software 17Information About Dynamic DNS Support for Cisco IOS Software 18
Domain Name System and Dynamic Updates 18
DDNS Updates for HTTP-Based Protocols 18
DHCP Support for DDNS Updates 18
Feature Design of Dynamic DNS Support for Cisco IOS Software 19
How to Configure Dynamic DNS Support for Cisco IOS Software 19
Configuring a Host List 20
Verifying the Host-List Configuration 21
Configuring DHCP Support of DDNS Updates 24
Configuring DDNS Update Support on Interfaces 26
Configuring a Pool of DHCPServers to Support DDNS Updates 28
Configuring the Update Method and Interval 30
Verifying DDNS Updates 34
Configuration Examples for Dynamic DNS Support for Cisco IOS Software 39
Configuration of the DHCP Client Example 39
Configuration of the DHCP Server Example 40
Configuration of the HTTP Updates Example 40
Additional References 42
Feature Information for Dynamic DNS Support for Cisco IOS Software 43
VRF-Aware DNS 45
Finding Feature Information 45
Information About VRF-Aware DNS 45
Domain Name System 45
VRF Mapping and VRF-Aware DNS 46
How to Configure VRF-Aware DNS 46
Defining a VRF Table and Assigning a Name Server to Enable VRF-Aware DNS 46
Mapping VRF-Specific Hostnames to IP Addresses 48
Configuring a Static Entry ina VRF-Specific Name Cache 49
Verifying the Name Cache Entries in the VRF Table 50
Configuration Examples for VRF-Aware DNS 51
VRF-Specific Name Server Configuration Example 51
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VRF-Specific Domain Name List Configuration Example 51
VRF-Specific Domain Name Configuration Example 52
VRF-Specific IP Host Configuration Example 52
Additional References 52Feature Information for VRF-Aware DNS 53
Split DNS 55
Finding Feature Information 55
Prerequisites for Split DNS 55
Restrictions for Split DNS 55
Information About Split DNS 56
Split DNS Feature Overview 56
Split DNS Use to Respond to DNS Queries Benefits 56
Selection of Virtual DNS Caching Name Server Configurations 56
Ability to Offload Internet Traffic from the Corporate DNS Server 57
Compatibility with NAT and PAT 57
Split DNS Operation 57
CPE Router Configuration 58
DNS Query Issued bya CPE Client 59
Virtual DNS Name Server Selection 59
Response to the Client-issued DNS Query 59
DNS Views 60
View Use Is Restricted toQueries from the Associated VRF 60
Parameters for ResolvingInternally Generated DNS Queries 61
Parameters for Forwarding Incoming DNS Queries 61
DNS View Lists 61
DNS Name Groups 63
DNS View Groups 63
Router Response to DNS Queries in a Split DNS Environment 64
Response to Incoming DNS Queries per the Forwarding Parameters of the Selected DNS
View 64
Response to Internally Generated DNS Queries per the Resolving Parameters of the
Default Global DNS View 65
How to Configure Split DNS 66
Enabling Split DNS Debugging Output 66
Defining a DNS Name List 68
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Defining a DNS View 69
Defining Static Entries in the Hostname Cache for a DNS View 73
Defining a DNS View List 75
Modifying a DNS View List 77Adding a Member to a DNS View List Already in Use 77
Changing the Order of the Members of a DNS View List Already in Use 78
Specifying the Default DNS View List for the DNS Server of the Router 80
Specifying a DNS View List for a Router Interface 81
Specifying a Source Interface to Forward DNS Queries 82
Configuration Examples for Split DNS 83
Split DNS View Limited to Queries from a Specific VRF Example 84
Split DNS View with Dynamic Name Server Configuration Example 84
Split DNS View with Statically Configured Hostname Cache Entries Example 85
Split DNS View with Round-Robin Rotation of Hostname Cache Entries Example 85
Split DNS Configuration of ACLs That Can Limit DNS View Use Example 85
Split DNS View Lists Configured with Different View-use Restrictions Example 86
Split DNS Configuration of Default and Interface-specific View Lists Example 87
Additional References 88
Feature Information for Split DNS 89
Glossary 89
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Configuring DNS
The Domain Name System (DNS) is a distributed database in which you can map hostnames to IP
addresses through the DNS protocol from a DNS server. Each unique IP address can have an associated
hostname. The Cisco IOS software maintains a cache of hostname-to-address mappings for use by the
connect, telnet, and pingEXEC commands, and related Telnet support operations. This cache speeds the
process of converting names to addresses.
Finding Feature Information, page 1 Prerequisites for Configuring DNS, page 1
Information About DNS, page 1
How to Configure DNS, page 3
Configuration Examples for DNS, page 13
Additional References, page 14
Feature Information for DNS, page 15
Finding Feature InformationYour software release may not support all the features documented in this module. For the latest feature
information and caveats, see the release notes for your platform and software release. To find informationabout the features documented in this module, and to see a list of the releases in which each feature is
supported, see the Feature Information Table at the end of this document.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support.
To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required
Prerequisites for Configuring DNSTo use DNS, you must have a DNS name server on your network.
Information About DNS DNS Overview, page 1
DNS OverviewIf your network devices require connectivity with devices in networks for which you do not control name
assignment, you can assign device names that uniquely identify your devices within the entire internetwork
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The global naming scheme of the Internet, the DNS, accomplishes this task. This service is enabled by
default. The following sections summarize DNS concepts and function.
Hostnames for Network Devices, page 2
Domains Names for Groups of Networks, page 2
Name Servers, page 2
Cache, page 2
Name Resolvers, page 2
Zones, page 3
Authoritative Name Servers, page 3
DNS Operation, page 3
Hostnames for Network Devices
Each unique IP address can have an associated hostname. DNS uses a hierarchical scheme for establishing
hostnames for network nodes. This allows local control of the segments of the network through a client-
server scheme. The DNS system can locate a network device by translating the hostname of the device into
its associated IP address.
Domains Names for Groups of Networks
IP defines a naming scheme that allows a device to be identified by its location in the IP. This is a
hierarchical naming scheme that provides for domains. On the Internet, a domain is a portion of the naming
hierarchy tree that refers to general groupings of networks based on organization type or geography.
Domain names are pieced together with periods (.) as the delimiting characters. For example, Cisco is a
commercial organization that the IP identifies by a comdomain name, so its domain name is cisco.com. A
specific device in this domain, the File Transfer Protocol (FTP) system, for example, is identified as
ftp.cisco.com.
Name ServersTo keep track of domain names, IP has defined the concept of a nameserver. Name servers are programs
that have complete information about their namespace portion of the domain tree and may also contain
pointers to other name servers that can be used to lead to information from any other part of the domain
tree. Name servers know the parts of the domain tree for which they have complete information. A name
server may also store information about other parts of the domain tree. Before domain names can be
mapped to IP addresses, you must first identify the hostnames, then specify a name server, and enable the
DNS service.
Cache
To speed the process of converting names to addresses, the name server maintains a database, called a
cache, of hostname-to-address mappings for use by the connect, telnet, and pingEXEC commands, and
related Telnet support operations. The cache stores the results from previous responses. Upon receiving a
client-issued DNS query, the name server will check this local storage to see if the answer is available
locally.
Name Resolvers
Name resolvers are programs that extract information from name servers in response to client requests.
Resolvers must be able to access at least one name server. The resolver either uses that name server's
Configuring DNS
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information to answer a query directly or pursues the query using referrals to other names servers. A
resolver will typically be a system routine that is directly accessible to user programs. Therefore, no
protocol is necessary between the resolver and the user program.
Zones
The domain namespace is divided into areas called zones that are points of delegation in the DNS tree. A
zone contains all domains from a certain point downward, except those for which other zones are
authoritative.
Authoritative Name Servers
A name server is said to be an authority for the parts ofthe domain tree for which it has complete
information. A zone usually has an authoritative name server, often more than one. An authoritative name
server has been configured with host table information or has acquired host table information though a zone
transfer (the action that occurs when a secondary DNS server starts up and updates itself from the primary
server).
DNS Operation
An organization can have many name servers, but Internet clients can query only those that the root name
servers know. The other name servers answer internal queries only.
A name server handles client-issued queries to the DNS server for locally defined hosts within a particular
zone as follows:
An authoritative name server responds to DNS user queries for a domain name that is under its zone of
authority by using the permanent and cached entries in its own host table. If the query is for a domain
name that is under its zone of authority but for which it does not have any configuration information,
the authoritative name server simply replies that no such information exists.
A name server that is not configured as the authoritative name server responds to DNS user queries by
using information that it has cached from previously received query responses. If no router isconfigured as the authoritative name server for a zone, queries to the DNS server for locally defined
hosts will receive nonauthoritative responses.
Name servers answer DNS queries (forward incoming DNS queries or resolve internally generated DNS
queries) according to the forwarding and lookup parameters configured for the specific domain.
When DNS queries are forwarded to name servers for resolution, some memory space is held for the
corresponding DNS query until an appropriate response is received or until there is timeout. To avoid the
free I/O memory from getting exhausted when handling queries at high rate, configure the maximum size
for the queue.
How to Configure DNS Mapping Hostnames to IP Addresses, page 4
Customizing DNS, page 5
Configuring DNS Spoofing, page 7
Configuring the Router as a DNS Server, page 8
Disabling DNS Queries for ISO CLNS Addresses, page 11
Verifying DNS, page 12
Configuring DNS
Zones
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Mapping Hostnames to IP AddressesPerform this task to map hostnames to IP addresses.
A name server is used to keep track of information associated with domain names. A name server can
maintain a database of hostname-to-address mappings. Each name can map to one or more IP addresses. Inorder to use this service to map domain names to IP addresses, you must specify a name server.
The name lookup system can be statically configured using the commands described in this task. Some
other functions in Cisco IOS software, such as DHCP, can dynamically modify the state of the name
lookup system. Use the showhostscommand to display the cached hostnames and the DNS configuration.
SUMMARY STEPS
1. enable
2. configureterminal
3. iphostname[tcp-port-number] address1[address2...address8]
4. Do one of the following:
ipdomainnamename
ipdomainlistname
5. ipname-serverserver-address1[server-address2...server-address6]
6. ipdomainlookup[source-interfaceinterface-typeinterface-number]
DETAILED STEPS
Command or Action Purpose
Step 1 enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2 configureterminal
Example:
Router# configure terminal
Enters global configuration mode.
Step 3 iphostname[tcp-port-number] address1
[address2...address8]
Example:
Router(config)# ip host cisco-rtp192.168.0.148
Defines a static hostname-to-address mapping in the hostname cache.
Typically, it is easier to refer to network devices by symbolic namesrather than numerical addresses (services such as Telnet can use
hostnames or addresses). Hostnames and IP addresses can be
associated with one another through static or dynamic means.
Manually assigning hostnames to addresses is useful when dynamic
mapping is not available.
Mapping Hostnames to IP Addresses
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Command or Action Purpose
Step 4 Do one of the following:
ipdomainnamename
ipdomainlistname
Example:
Router(config)# ip domain namecisco.com
Example:
Example:
Router(config)# ip domain listcisco1.com
(Optional) Defines a default domain name that the Cisco IOS software will
use to complete unqualified hostnames.
or
(Optional) Defines a list of default domain names to complete unqualified
hostnames.
You can specify a default domain name that the Cisco IOS software
will use to complete domain name requests. You can specify either a
single domain name or a list of domain names. Any hostname that does
not contain a complete domain name will have the default domain
name you specify appended to it before the name is looked up.
Note If there is no domain list, the domain name that you specified with
the ipdomainnameglobal configuration command is used. If there
is a domain list, the default domain name is not used. The ipdomain
listcommand is similar to the ipdomainnamecommand, exceptthat with the ipdomainlistcommand you can define a list of
domains, each to be tried in turn until the system finds a match.
Step 5 ipname-serverserver-address1[server-address2...server-address6]
Example:
Router(config)# ip name-server172.16.1.111 172.16.1.2
Specifies one or more hosts (up to six) that can function as a name server to
supply name information for DNS.
Step 6 ipdomainlookup[source-interfaceinterface-typeinterface-number]
Example:
Router(config)# ip domain lookup
(Optional) Enables DNS-based address translation.
DNS is enabled by default. Use this command if DNS has been
disabled.
Customizing DNSPerform this task to customize your DNS configuration.
In a multiple server configuration without the DNS round-robin functionality, many programs will use the
first host server/IP address for the whole time to live (TTL) of the cache and use the second and third host
servers/IP addresses only in the event of host failure. This behavior presents a problem when a high volume
of users all arrive at the first host during the TTL time. For example, the network access server (NAS)
sends out a DNS query. The DNS servers reply with a list of the configured IP addresses to the NAS. The
NAS then caches these IP addresses for a given time (for example, five minutes). All users that dial in
during the five minute TTL time will land on one host, the first IP address in the list.
In a multiple server configuration with the DNS round-robin functionality, the DNS server returns the IP
address of all hosts to rotate between the cache of hostnames. During the TTL of the cache, users are
Customizing DNS
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distributed among the hosts. This functionality distributes calls across the configured hosts and reduces the
number of DNS queries.
In a scheduling algorithm, processes are activated in a fixed cyclic order. Processes that are waiting for
other events, like termination of a child process or an input or output operation, cannot proceed and hence
they return control to the scheduler. If the TTL of the process times out just before the event (for which it
was waiting) occurs, then the event will not be handled until all the other processes are activated.
Note The DNS round-robin functionality is applicable only for the DNS lookups on a router and is not applicable
to another client pointing to the router.
SUMMARY STEPS
1. enable
2. configureterminal
3. ipdomaintimeoutseconds
4. ipdomainretrynumber
5. ipdomainround-robin
DETAILED STEPS
Command or Action Purpose
Step 1 enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2 configureterminal
Example:
Router# configure terminal
Enters global configuration mode.
Step 3 ipdomaintimeoutseconds
Example:
Router(config)# ip domain timeout 17
(Optional) Specifies the amount of time to wait for a response to a DNS
query.
If the ipdomaintimeoutcommand is not configured, the Cisco IOS
software will wait 3 seconds for a response to a DNS query.
Step 4 ipdomainretrynumber
Example:
Router(config)# ip domain retry 10
(Optional) Specifies the number of times to retry sending DNS queries.
If the ipdomainretrycommand is not configured, the Cisco IOS
software will retry DNS queries twice.
Configuring DNS
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Command or Action Purpose
Step 5 ipdomainround-robin
Example:
Router(config)# ip domain round-robin
(Optional) Enables round-robin functionality on DNS servers.
Configuring DNS SpoofingPerform this task to configure DNS spoofing.
DNS spoofing is designed to allow a router to act as a proxy DNS server and spoof replies to any DNS
queries using either the configured IP address in the ipdnsspoofingip-addresscommand or the IP address
of the incoming interface for the query. This feature is useful for devices where the interface toward the
Internet service provider (ISP) is not up. Once the interface to the ISP is up, the router forwards DNS
queries to the real DNS servers.
This feature turns on DNS spoofing and is functional if any of the following conditions are true:
The noipdomainlookupcommand is configured.
IP name server addresses are not configured.
There are no valid interfaces or routes for sending to the configured name server addresses.
If these conditions are removed, DNS spoofing will not occur.
SUMMARY STEPS
1. enable
2. configureterminal
3. ipdnsserver
4. ipdnsspoofing[ip-address]
DETAILED STEPS
Command or Action Purpose
Step 1 enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2 configureterminal
Example:
Router# configure terminal
Enters global configuration mode.
Configuring DNS Spoofing
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Command or Action Purpose
Step 3 ipdnsserver
Example:
Router(config)# ip dns server
Activates the DNS server on the router.
Step 4 ipdnsspoofing[ip-address]
Example:
Router(config)# ip dns spoofing192.168.15.1
Configures DNS spoofing.
The router will respond to the DNS query with the configured ip-
addresswhen queried for any hostname other than its own.
The router will respond to the DNS query with the IP address of
the incoming interface when queried for its own hostname.
Configuring the Router as a DNS ServerPerform this task to configure the router as a DNS server.
A Cisco IOS router can provide service to DNS clients, acting as both a caching name server and as an
authoritative name server for its own local host table.
When configured as a caching name server, the router relays DNS requests to other name servers that
resolve network names into network addresses. The caching name server caches information learned from
other name servers so that it can answer requests quickly, without having to query other servers for each
transaction.
When configured as an authoritative name server for its own local host table, the router listens on port 53
for DNS queries and then answers DNS queries using the permanent and cached entries in its own host
table.
An authoritative name server usually issues zone transfers or responds to zone transfer requests from other
authoritative name servers for the same zone. However, the Cisco IOS DNS server does not perform zone
transfers.
When it receives a DNS query, an authoritative name server handles the query as follows:
If the query is for a domain name that is not under its zone of authority, the authoritative name server
determines whether to forward the query to specific back-end name servers based on whether IP DNS-
based hostname-to-address translation has been enabled via the ipdomainlookupcommand.
If the query is for a domain name that is under its zone of authority and for which it has configuration
information, the authoritative name server answers the query using the permanent and cached entries
in its own host table.
If the query is for a domain name that is under its zone of authority but for which it does not have any
configuration information, the authoritative name server does not forward the query elsewhere for aresponse; instead the authoritative name server simply replies that no such information exists.
Configuring the Router as a DNS Server
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Note Unless Distributed Director is enabled, the TTL on locally defined resource records will always be ten
seconds, regardless of any authority record parameters that may have been specified for the DNS name
server by the use of the ipdnsprimarycommand.
>
SUMMARY STEPS
1. enable
2. configureterminal
3. ipdnsserver
4. ipname-serverserver-address1[server-address2... server-address6]
5. ipdnsserverqueuelimit{forwarderqueue-size-limit| directorqueue-size-limit}
6. iphost[vrfvrf-name] [viewview-name] hostname{address1[address2... address8] | additionaladdress9[address10... addressn]}
7. ipdnsprimarydomain-namesoaprimary-server-namemailbox-name[refresh-interval[retry-interval
[expire-ttl[minimum-ttl]]]]
8. iphostdomain-namensserver-name
DETAILED STEPS
Command or Action Purpose
Step 1 enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2 configureterminal
Example:
Router# configure terminal
Enters global configuration mode.
Step 3 ipdnsserver
Example:
Router(config)# ip dns server
Enables the DNS server.
Configuring DNS
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Command or Action Purpose
Step 4 ipname-serverserver-address1[server-address2...
server-address6]
Example:
Router(config)# ip name-server 192.168.2.120192.168.2.121
(Optional) Configures other DNS servers:
Cisco IOS resolver name servers
DNS server forwarders
Note If the Cisco IOS name server is being configured to
respond only to domain names for which it is
authoritative, there is no need to configure other DNS
servers.
Step 5 ipdnsserverqueuelimit{forwarderqueue-size-limit|
directorqueue-size-limit}
Example:
Router(config)# ip dns server queue limitforwarder 10
(Optional) Configures a limit to the size of the queues used by
the DNS server processes.
The directorkeyword was removed in Cisco IOS
Release 12.4(24)T.
Step 6 iphost[vrfvrf-name] [viewview-name] hostname
{address1[address2... address8] | additionaladdress9
[address10... addressn]}
Example:
Router(config)# ip host user1.example.com192.168.201.5 192.168.201.6
(Optional) Configures local hosts.
Step 7 ipdnsprimarydomain-namesoaprimary-server-name
mailbox-name[refresh-interval[retry-interval[expire-ttl
[minimum-ttl]]]]
Example:
Router(config)# ip dns primary example.com soans1.example.com mb1.example.com
Configures the router as the primary DNS name server for a
domain (zone) and as the start of authority (SOA) record
source (which designates the start of a zone).
Note Unless Distributed Director is enabled, the TTL on
locally defined resource records will always be ten
seconds.
Step 8 iphostdomain-namensserver-name
Example:
Router(config)# ip host example.com ns
ns1.example.com
(Optional) Configures the router to create an name server
(NS) resource record to be returned when the DNS server is
queried for the associated domain.
This configuration is needed only if the zone for which
the system is authoritative will also be served by other
name servers.
Examples, page 10
Examples
This section provides examples of debugging output that is logged when a router is configured as an
authoritative name server for its own local host table and the debugdomaincommand is in effect:
Configuring DNS
Examples
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Note For DNS-based X.25 routing, the debugx25eventscommand supports functionality to describe the event
that occur while the X.25 address is being resolved to an IP address using a DNS server. The debug
domaincommand can be used along with debugx25eventsto observe the whole DNS-based X.25 routin
data flow.
Debugging Output for Relaying a DNS Query to Another Name Server Example, page 11
Debugging Output for Servicing a DNS Query from the Local Host Table Example, page 11
Debugging Output for Relaying a DNS Query to Another Name Server Example
The following is sample output from the debugdomaincommand that corresponds to relaying a DNS
query to another name server when the router is configured as an authoritative name server for its own local
host table:
Apr 4 22:18:32.183: DNS: Incoming UDP query (id#18713)Apr 4 22:18:32.183: DNS: Type 1 DNS query (id#18713) for host 'ns1.example.com' from192.0.2.120(1283)Apr 4 22:18:32.183: DNS: Re-sending DNS query (type 1, id#18713) to 192.0.2.121Apr 4 22:18:32.211: DNS: Incoming UDP query (id#18713)Apr 4 22:18:32.211: DNS: Type 1 response (id#18713) for host from192.0.2.121(53)Apr 4 22:18:32.215: DOM: dom2cache: hostname is ns1.example.com, RR type=1, class=1,ttl=86400, n=4Apr 4 22:18:32.215: DNS: Forwarding back A response - no director requiredApr 4 22:18:32.215: DNS: Finished processing query (id#18713) in 0.032 secsApr 4 22:18:32.215: DNS: Forwarding back reply to 192.0.2.120/1283
Debugging Output for Servicing a DNS Query from the Local Host Table Example
The following is sample output from the debugdomaincommand that corresponds to servicing a DNS
query from the local host table when the router is configured as an authoritative name server for its own
local host table:
Apr 4 22:16:35.279: DNS: Incoming UDP query (id#8409)Apr 4 22:16:35.279: DNS: Type 1 DNS query (id#8409) for host 'ns1.example.com' from192.0.2.120(1279)Apr 4 22:16:35.279: DNS: Finished processing query (id#8409) in 0.000 secs
Disabling DNS Queries for ISO CLNS AddressesPerform this task to disable DNS queries for International Organization for Standardization (ISO)
Connectionless Network Service (CLNS) addresses.
If your router has both IP and ISO CLNS enabled and you want to use ISO CLNS network service access
point (NSAP) addresses, you can use the DNS to query these addresses, as documented in RFC 1348. This
feature is enabled by default.
SUMMARY STEPS
1. enable
2. configureterminal
3. noipdomainlookupnsap
Disabling DNS Queries for ISO CLNS Addresses
Debugging Output for Relaying a DNS Query to Another Name Server Example
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DETAILED STEPS
Command or Action Purpose
Step 1 enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2 configureterminal
Example:
Router# configure terminal
Enters global configuration mode.
Step 3 noipdomainlookupnsap
Example:
Router(config)# no ip domain lookup nsap
Disables DNS queries for ISO CLNS addresses.
Verifying DNSPerform this task to verify your DNS configuration.
1 enable
2 pinghosts
3 showhosts
SUMMARY STEPS
1. enable
2. pinghosts
3. showhosts
DETAILED STEPS
Command or Action Purpose
Step 1 enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Verifying DNS
Debugging Output for Servicing a DNS Query from the Local Host Table Example
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Command or Action Purpose
Step 2 pinghosts
Example:
Router# ping cisco-rtp
Diagnoses basic network connectivity.
After the DNS configuration is set, you can verify the DNS server by using a hostname to
ping or telnet to a device.
Step 3 showhosts
Example:
Router# show hosts
Displays the default domain name, the style of name lookup service, a list of name server
hosts, and the cached list of hostnames and addresses.
After a name is resolved using DNS, use the showhostscommand to view the cached
hostnames and the DNS configuration.
Configuration Examples for DNS IP Addresses Example, page 13
Mapping Hostnames to IP Addresses Example, page 13
Customizing DNS Example, page 13
Configuring DNS Spoofing Example, page 14
IP Addresses ExampleThe following example establishes a domain list with several alternate domain names:
ip domain list example.comip domain list example1.eduip domain list example2.edu
Mapping Hostnames to IP Addresses ExampleThe following example configures the hostname-to-address mapping process. IP DNS-based translation is
specified, the addresses of the name servers are specified, and the default domain name is given.
! IP DNS-based hostname-to-address translation is enabledip domain lookup! Specifies hosts 192.168.1.111 and 192.168.1.2 as name serversip name-server 192.168.1.111 192.168.1.2! Defines cisco.com as the default domain name the router uses to complete! Set the name for unqualified hostnames
ip domain name cisco.com
Customizing DNS ExampleThe following example allows a Telnet to company.example.com to connect to each of the three IP
addresses specified in the following order: the first time the hostname is referenced, it would connect to
10.0.0.1; the second time the hostname is referenced, it would connect to 10.1.0.1; and the third time the
IP Addresses Example
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hostname is referenced, it would connect to 10.2.0.1. In each case, the other two addresses would also be
tried if the first one failed; this is the normal operation of the Telnet command.
Router(config)# ip host company.example.com 10.0.0.1 10.1.0.1 10.2.0.1Router(config)# ip domain round-robin
Configuring DNS Spoofing ExampleIn the following example, the router is configured to spoof replies to any DNS queries:
ip dns serverip dns spoofingno ip domain lookupinterface e3/1ip address 10.1.1.1 255.255.255.0
Additional References
Related Documents
Related Topic Document Title
DNS commands: complete command syntax,
command mode, command history, defaults, usage
guidelines, and examples
Cisco IOS IP Addressing Services Command
Reference
Standards
Standards Title
No new or modified standards are supported by thisfunctionality.
--
MIBs
MIBs MIBs Link
No new or modified MIBs are supported by this
feature, and support for existing MIBs has not been
modified by this feature.
To locate and download MIBs for selected
platforms, Cisco IOS releases, and feature sets, use
Cisco MIB Locator found at the following URL:
http://www.cisco.com/go/mibs
RFCs
RFCs Title
RFC 1348 DNS NSAP RRs
Configuring DNS Spoofing Example
Additional References
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Technical Assistance
Description Link
The Cisco Support website provides extensive
online resources, including documentation and tools
for troubleshooting and resolving technical issueswith Cisco products and technologies.
To receive security and technical information about
your products, you can subscribe to various
services, such as the Product Alert Tool (accessed
from Field Notices), the Cisco Technical Services
Newsletter, and Really Simple Syndication (RSS)
Feeds.
Access to most tools on the Cisco Support website
requires a Cisco.com user ID and password.
http://www.cisco.com/techsupport
Feature Information for DNSThe following table provides release information about the feature or features described in this module.
This table lists only the software release that introduced support for a given feature in a given software
release train. Unless noted otherwise, subsequent releases of that software release train also support that
feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support.
To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required
Table 1 Feature Information for DNS
Feature Name Releases Feature Information
DNS Spoofing 12.3(2)T This feature is designed to allow
a router to act as a proxy DNS
server and spoof replies to any
DNS queries using either the
configured IP address in the ip
dnsspoofingip-address
command or the IP address of the
incoming interface for the query.
The following command was
introduced by this feature: ipdns
spoofing.
Cisco and the Cisco logo are trademarks or registered trademarks of Cisco and/or its affiliates in the U.S.
and other countries. To view a list of Cisco trademarks, go to this URL: www.cisco.com/go/trademarks.
Configuring DNS
Feature Information for DNS
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Third-party trademarks mentioned are the property of their respective owners. The use of the word partner
does not imply a partnership relationship between Cisco and any other company. (1110R)
Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be
actual addresses and phone numbers. Any examples, command display output, network topology diagrams,
and other figures included in the document are shown for illustrative purposes only. Any use of actual IP
addresses or phone numbers in illustrative content is unintentional and coincidental.
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Dynamic DNS Support for Cisco IOS Software
The Dynamic DNS Support for Cisco IOS Software feature enables Cisco IOS software devices to
perform Dynamic Domain Name System (DDNS) updates to ensure that an IP host DNS name is correctly
associated with its IP address.
It provides two mechanisms to generate or perform DDNS: the IETF standard as defined by RFC 2136
and a generic HTTP using various DNS services. Withthis feature, you can define a list of hostnames and
IP addresses that will receive updates, specify an update method, and specify a configuration for Dynamic
Host Configuration Protocol (DHCP) triggered updates.
Finding Feature Information, page 17
Restrictions for Dynamic DNS Support for Cisco IOS Software, page 17
Information About Dynamic DNS Support for Cisco IOS Software, page 18
How to Configure Dynamic DNS Support for Cisco IOS Software, page 19
Configuration Examples for Dynamic DNS Support for Cisco IOS Software, page 39
Additional References, page 42
Feature Information for Dynamic DNS Support for Cisco IOS Software, page 43
Finding Feature InformationYour software release may not support all the features documented in this module. For the latest featureinformation and caveats, see the release notes for your platform and software release. To find information
about the features documented in this module, and to see a list of the releases in which each feature is
supported, see the Feature Information Table at the end of this document.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support.
To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required
Restrictions for Dynamic DNS Support for Cisco IOS SoftwareThe performance of the DHCP client can be impacted when the Dynamic DNS Support for Cisco IOS
Software feature is enabled, because of sending DDNS update packets and waiting for responses from theserver (before sending the ACK to the client REQUEST) and the client (immediately after receiving the
ACK and assigning the address to the interface). The default for the client is two attempts with a 5-second
wait time between attempts.
The DHCP server continues to process DHCP client DISCOVER and REQUEST packets while waiting for
the DDNS updates to complete. Even if the update is done before sending the ACK to the client, it does not
delay processing of other DHCP requests. The DHCP server could be impacted minimally because of the
time and memory needed in order to set up the DDNS update and get things started.
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Reloading the system may take a little longer in some cases, such as, if there are outstanding DDNS
updates that need to complete.
Information About Dynamic DNS Support for Cisco IOSSoftware
Domain Name System and Dynamic Updates, page 18
DDNS Updates for HTTP-Based Protocols, page 18
DHCP Support for DDNS Updates, page 18
Feature Design of Dynamic DNS Support for Cisco IOS Software, page 19
Domain Name System and Dynamic UpdatesThe DNS was designed to support queries of a statically configured database. The data was expected to
change, but minimally. All updates were made as external edits to a zone master file. The domain nameidentifies a node within the domain name space tree structure. Each node has a set (possibly empty) of
Resource Records (RRs). All RRs having the same NAME, CLASS, and TYPE are called a Resource
Record Set (RRset).
There are address (A) or forward RRs and pointer (PTR) or reverse RRs. The DDNS update can specify
additions or deletions of hostnames and IP addresses. The two mechanisms to update this information are
by using HTTP-based protocols such as DynDNS.org or by using the IETF standard.
DDNS Updates for HTTP-Based ProtocolsThe Dynamic DNS Support for Cisco IOS Software feature provides the capability of a proprietary HTTP-
based protocol to generate or perform DDNS updates. The most notable HTTP-based protocol is
DynDNS.org, but there are many others.
Since most of these protocols consist of a simple HTTP command that specifies parameters such as
hostname and IP address in the URL portion of the command, this feature takes the same generic approach.
You can specify the hostname and IP address in a URL. Configuration of a maximum interval between
updates is also allowed.
DHCP Support for DDNS UpdatesBefore the Dynamic DNS Support for Cisco IOS Software feature, a DHCP server assigned IP addresses to
DHCP clients and any DNS information was static. In a network that uses a DHCP server, there are many
cases in which DNS hostnames should be associated with the IP addresses that are being assigned. There is
an existing method for dynamically updating DNS for DHCP by using information in the fully qualified
domain name (FQDN) DHCP option (if it is supplied by the client).
The Dynamic DNS Support for Cisco IOS Software feature enables the DHCP server to support a new
FQDN DHCP option. In addition, when the address on an interface is configured, the client can pass the
new FQDN option to the server so that name-to-address and address-to-name translations can be updated
for the DHCP client as well.
Domain Name System and Dynamic Updates
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Feature Design of Dynamic DNS Support for Cisco IOS SoftwareThe Dynamic DNS Support for Cisco IOS Software feature enables the tracking of the FQDN DHCP
option. If dynamic updates are enabled for the DHCP server, the server updates the PTR RR. The PTR RRs
are used for reverse mapping (translation of addresses to names). PTRs use official names not aliases. Thename in a PTR record is the local IP address portion of the reverse name.
If the client requests the server to update A RRs as well,the server will attempt to do it. The A RR provides
the name-to-address mapping for a DNS zone. The server may be configured to override the client
suggestion and always update PTR and A RRs.
The DHCP client can specify whether or not it wants to allow dynamic updates (include the FQDN option),
instruct the server to allow the client to update both A and PTR RRs (normally only the A RR is updated by
the client), and optionally instruct the server not to update any DNS information (either because the client
will be updating both or simply because the client does not want the server to do any updates at all).
There are three basic components of the Dynamic DNS Support for Cisco IOS Software feature that are as
follows:
Definition of the hostname list and IP addresses that will receive updates using a new command thatspecifies a group of hostnames. Each configured list can consist of any number of IPv4 addresses or
hostnames. If a hostname is configured, the name is translated to an IPv4 address at the time at which
it is used.
Specification of an update method. The options are HTTP, DDNS, or an internal Cisco IOS name
cache. If the HTTP option is specified, the configuration will include a URL. The username and
password must be explicitly written into the URL string and the entire GET operation must be
specified on one line. The specification will be stored in a linked list. If the update method is DDNS,
the configuration will include the update of the IP address.
Events that trigger updates can be as follows:
IP address that is assigned by a DHCP server for an IP device
IP address assigned to a router using a DHCP client
Forwarding of the fully qualified domain name (FQDN) of a user or router hostname from the DHCP
client to the server
Point-to-Point Protocol (PPP)/IP Control Protocol (IPCP) obtaining an IP address for a router interface
Forced update using a timer to verify a router IP address
Associated with each update method is a value specifying the maximum number of seconds between
updates. If left unspecified, then the update is performed only when the address is changed. If specified, the
update is performed automatically if the specified number of seconds have passed since the last update.
How to Configure Dynamic DNS Support for Cisco IOS
Software
Note The internal Cisco IOS name cache does not require any configuration.
Configuring a Host List, page 20
Verifying the Host-List Configuration, page 21
Feature Design of Dynamic DNS Support for Cisco IOS Software
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Configuring DHCP Support of DDNS Updates, page 24
Configuring DDNS Update Support on Interfaces, page 26
Configuring a Pool of DHCP Servers to Support DDNS Updates, page 28
Configuring the Update Method and Interval, page 30
Verifying DDNS Updates, page 34
Configuring a Host ListPerform this task to configure a host list if you are going to use a host list in your configuration.
SUMMARY STEPS
1. enable
2. configureterminal
3. iphost-listhost-list-name
4. host[vrfvrf-name] {host-ip-address| hostname}
5. exit
DETAILED STEPS
Command or Action Purpose
Step 1 enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2 configureterminal
Example:
Router# configure terminal
Enters global configuration mode.
Step 3 iphost-listhost-list-name
Example:
Router(config)# ip host-list abc
Specifies a list of hosts and enters host-list configuration mode. The
host-list-nameargumentassigns a name to the list of hosts.
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Command or Action Purpose
Step 4 host[vrfvrf-name] {host-ip-address|
hostname}
Example:
Router(host-list)# host 10.1.1.110.2.2.2 10.3.3.3 a.com b.com 10.4.4.410.5.5.5 d.com host 10.6.6.6 f.com hostvrf abc a.com b.com c.com host vrf def10.1.1.1 10.2.2.2 10.3.3.3
Configures one or more hosts. The arguments and keyword are as
follows:
vrfvrf-name--Associates a hostname with a virtual privatenetwork (VPN) routing and forwarding instance (VRF) name.
Note All hostnames or IP addresses specified after the vrfkeyword
are associated with that VRF.
host-ip-address--Specifies an IP address for a host in the host list.
You can specify more than one host using this argument by listing
the hostname and IP addresses on the same line.
hostname--Specifies a hostname.
Step 5 exit
Example:
Router(host-list)# exit
Exits to global configuration mode.
Examples
The following example shows how to configure several hosts with VRF:
ip host-list abchost 10.1.1.1 10.2.2.2 10.3.3.3 a.com b.com 10.4.4.4 10.5.5.5 d.comhost 10.6.6.6 f.comhost vrf abc a.com b.com c.comhost vrf def 10.1.1.1 10.2.2.2 10.3.3.3
Verifying the Host-List ConfigurationTo verify the host-list configuration, perform the following steps.
SUMMARY STEPS
1. showiphost-list
2. showrunning-config|inchost-list
3. showrunning-config|inchost
4. debugipddnsupdate
DETAILED STEPS
Step 1 showiphost-listUse this command to verify that the IP addresses and hostnames have been assigned to a host list, for example:
Example:
Router# show ip host-list abc
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Host list: abcddns.abc10.2.3.4ddns2.abc10.3.4.5ddns3.com
10.3.3.3d.orge.org1.org.2.org3.com10.2.2.2 (VRF: test)10.5.5.5 (VRF: test)a.net (VRF: test)b.net (VRF: test)
Step 2 showrunning-config|inchost-listUse this command to verify the configuration of a host list, for example:
Example:
Router# show running-config | inc host-list
ip host-list aip host-list bip host-list cip host-list abc
Step 3 showrunning-config|inchostUse this command to verify the configuration of a hostname, for example:
Example:
Router# show running-config | inc hosthostname whoip host who 10.0.0.2ip host-list ahost 10.1.1.1 a.com b.com 10.2.2.3 10.2.2.2 c.com. 10.3.3.3 10.4.4.4host d.comhost vrf abc 10.10.10.4 10.10.10.8host vrf def 10.2.3.4 10.6.7.8ip host-list bhost a.com b.com c.com 10.1.1.1 10.2.2.2 10.3.3.3host vrf ppp 10.2.1.0ip host-list chost 10.1.1.1 10.2.2.2 10.3.3.3 a.com b.com 10.4.4.4 10.5.5.5 d.comhost 10.6.6.6 f.comhost vrf zero a.com b.com c.omhost vrf one 10.1.1.1 10.2.2.2 10.3.3.3ip host-list unit-testhost ddns.unit.test 10.2.3.4 ddns2.unit.test 10.3.4.5 ddns3.com 10.3.3.3 d.org e.orghost 1.org.2.org 3.comhost vrf ZERO 10.2.2.2 10.5.5.5 a.net b.netip ddns update hostname use-this.host.nameip ddns update this-method host 10.2.3.4ip ddns update this-method host this-hostip ddns update this-method host-group this-list
ip ddns update this-method host 10.3.4.5ip ddns update test host 10.19.192.32ip ddns update test host 10.19.192.32ip ddns update a host-group aip ddns update a host-group abip ddns update aa host-group abip ddns update method host 10.33.44.55
Step 4 debugipddnsupdate
Use the debugipddnsupdatecommand for the following configuration to verify the configuration of the hosts. Two
servers are configured in the host list. A DHCP client is configured for IETF DDNS updating of both A and DNS RRs
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and requesting the DHCP server to update neither. The DHCP client is configured to include an FQDN DHCP option
that instructs the DHCP server not to update either A or PTR Resource Records. This is configured using the interface
version of the command. The DHCP server is configured to allow the DHCP client to update whatever RRs it
chooses.
Example:
!Configure the DHCP Clientip host-list servershost 10.19.192.32 10.0.0.1ip ddns update method testingddnsinterface Ethernet1ip dhcp client update dns server noneip ddns update testing host-group serversip address dhcpend!Configure the DHCP Serverip dhcp pool testnetwork 10.0.0.0 255.0.0.0
update dns!Enable Debuggingdebug ip ddns update!The update to the server 10.0.0.1 fails in this example00:18:58:%DHCP-6-ADDRESS_ASSIGN: Interface Ethernet1 assigned DHCP address 10.0.0.8, mask255.0.0.0, hostname canada_reserved00:18:58: DYNDNSUPD: Adding DNS mapping for canada_reserved.hacks 10.0.0.8 server 10.19.192.3200:18:58: DYNDNSUPD: Sleeping for 3 seconds waiting for interface Ethernet1 configuration to settle00:19:01: DDNS: Enqueuing new DDNS update 'canada_reserved.hacks' 10.0.0.8 server 10.19.192.3200:19:01: DYNDNSUPD: Adding DNS mapping for canada_reserved.hacks 10.0.0.8 server 10.0.0.100:19:01: DDNS: Enqueuing new DDNS update 'canada_reserved.hacks' 10.0.0.8 server 10.0.0.100:19:01: DYNDNSUPD: Adding DNS mapping for canada_reserved.hacks 10.0.0.8 server 10.0.0.100:19:01: DDNS: Enqueuing new DDNS update 'canada_reserved.hacks' 10.0.0.8 server 10.0.0.100:19:01: DDNS: Zone name for '10.0.0.11.in-addr.arpa.' is '10.in-addr.arpa'00:19:01: DDNS: Using server 10.19.192.3200:19:01: DDNS: Dynamic Update 1: (sending to server 10.19.192.32)00:19:01: DDNS: Zone = 10.in-addr.arpa00:19:01: DDNS: Prerequisite: 10.0.0.11.in-addr.arpa. not in use
00:19:01: DDNS: Update: add 10.0.0.11.in-addr.arpa. IN PTR canada_reserved.hacks00:19:01: DDNS: Zone name for '10.0.0.11.in-addr.arpa.' is '10.in-addr.arpa'00:19:01: DDNS: Using server 10.0.0.100:19:01: DDNS: Dynamic Update 1: (sending to server 10.0.0.1)00:19:01: DDNS: Zone = 10.in-addr.arpa00:19:01: DDNS: Prerequisite: 10.0.0.11.in-addr.arpa. not in use00:19:01: DDNS: Update: add 10.0.0.11.in-addr.arpa. IN PTR canada_reserved.hacks00:19:01: DDNS: Zone name for '10.0.0.11.in-addr.arpa.' is '10.in-addr.arpa'00:19:01: DDNS: Using server 10.0.0.100:19:01: DDNS: Dynamic Update 1: (sending to server 10.0.0.1)00:19:01: DDNS: Zone = 10.in-addr.arpa00:19:01: DDNS: Prerequisite: 10.0.0.11.in-addr.arpa. not in use00:19:01: DDNS: Update: add 10.0.0.11.in-addr.arpa. IN PTR canada_reserved.hacks00:19:01: DDNS: Dynamic DNS Update 1 (PTR) for host canada_reserved.hacks returned 6 (YXDOMAIN)00:19:01: DDNS: Dynamic Update 2: (sending to server 10.19.192.32)00:19:01: DDNS: Zone = 10.in-addr.arpa00:19:01: DDNS: Update: delete 10.0.0.11.in-addr.arpa. all PTR RRs00:19:01: DDNS: Update: add 10.0.0.11.in-addr.arpa. IN PTR canada_reserved.hacks
00:19:01: DDNS: Dynamic DNS Update 2 (PTR) for host canada_reserved.hacks returned 0 (NOERROR)00:19:01: DDNS: Zone name for 'canada_reserved.hacks' is 'hacks'00:19:01: DDNS: Using server 10.19.192.3200:19:01: DDNS: Dynamic Update 1: (sending to server 10.19.192.32)00:19:01: DDNS: Zone = hacks00:19:01: DDNS: Prerequisite: canada_reserved.hacks not in use00:19:01: DDNS: Update: add canada_reserved.hacks IN A 10.0.0.800:19:01: DDNS: Dynamic DNS Update 1 (A) for host canada_reserved.hacks returned 0 (NOERROR)00:19:01: DDNS: Update of 'canada_reserved.hacks' 10.0.0.8 finished00:19:01: DYNDNSUPD: Another update completed (total outstanding=2)00:19:11: DDNS: Dynamic DNS Update 1 (PTR) for host canada_reserved.hacks returned 0 (NOERROR)00:19:11: DDNS: Dynamic DNS Update 1 (PTR) for host canada_reserved.hacks returned 0 (NOERROR)
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00:19:11: DDNS: Zone name for 'canada_reserved.hacks' is 'hacks'00:19:11: DDNS: Using server 10.0.0.100:19:11: DDNS: Dynamic Update 1: (sending to server 10.0.0.1)00:19:11: DDNS: Zone = hacks00:19:11: DDNS: Prerequisite: canada_reserved.hacks not in use00:19:11: DDNS: Update: add canada_reserved.hacks IN A 10.0.0.8
00:19:11: DDNS: Zone name for 'canada_reserved.hacks' is 'hacks'00:19:11: DDNS: Using server 10.0.0.100:19:11: DDNS: Dynamic Update 1: (sending to server 10.0.0.1)00:19:11: DDNS: Zone = hacks00:19:11: DDNS: Prerequisite: canada_reserved.hacks not in use00:19:11: DDNS: Update: add canada_reserved.hacks IN A 10.0.0.800:19:21: DDNS: Dynamic DNS Update 1 (A) for host canada_reserved.hacks returned 0 (NOERROR)00:19:21: DDNS: Update of 'canada_reserved.hacks' 10.0.0.8 failed00:19:21: DYNDNSUPD: Another update completed (total outstanding=1)00:19:21: DDNS: Dynamic DNS Update 1 (A) for host canada_reserved.hacks returned 0 (NOERROR)00:19:21: DDNS: Update of 'canada_reserved.hacks' 10.0.0.8 failed00:19:21: DYNDNSUPD: Another update completed (total outstanding=0)
Configuring DHCP Support of DDNS UpdatesDDNS updates contain information about A or forward RRs for a particular IP address. The IP address is in
dotted decimal form, and there must be at least one A record for each host address. The name specified is
the hostname expressed as an FQDN (ns.example.com). The PTR or reverse RRs map a domain name to
another domain name and is used for reverse mapping (IP address to domain name).
The updates are performed using messages. In general, you will probably want DDNS updates done by the
server after the server has sent the ACK response to the DHCP client. Performing the DDNS updates
before sending the ACK response will delay the response to the client. Both methods are supported. The
default is to do the updates after sending the response.
When looking for a client hostname to use in the update, the server will take the hostname from the FQDN
option, if such exists, first. If there is no FQDN option, the server will look for a HOSTNAME option and
take the name from there.
If the FQDN or HOSTNAME option is included in subsequent RENEWAL messages, the server will
attempt to perform the DDNS update each time the lease is renewed. This process gives the opportunity for
the client to change the name specified after the lease has been granted and have the server do the
appropriate updates. Although the server has this capability, the DHCP client will continue to use the same
hostname throughout the duration of a lease.
The IP address of the server to update is discovered by sending a DNS query for records associated with the
hostname to update. If such a record exists, the hostname of the master DNS server is extracted from this
information. If no such record exists, the record, which should be included in the response, is used as the
authoritative record for the zone where the hostname exists. In either case, once the master DNS server
hostname is found, another query for A RRs is sent in order to discover the IP address of this server. The
resulting IP address is used for sending updates.
Perform this task to configure the DDNS updates.
In order for DDNS updates to discover the DNS server, in cases in which the user did not configure the
server, the ipname-servercommand should be configured. This name server should be reachable by the
system, and the ipdomainlookupcommand should be configured (which is the default anyway). In cases
in which the configured hostname does not include a period (is not a fully qualified domain name
[FQDN]), an IP domain name should be configured.
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Note DHCP server-pool configuration commands and interface configurations have precedence over global
configurations.
SUMMARY STEPS
1. enable
2. configureterminal
3. ipdhcpupdatedns[both] [override] [before]
4. ipdhcp-clientupdatedns[server{both| none}]
5. exit
DETAILED STEPS
Command or Action Purpose
Step 1 enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2 configureterminal
Example:
Router# configure terminal
Enters global configuration mode.
Step 3 ipdhcpupdatedns[both][override] [before]
Example:
Router(config)# ip dhcpupdate dns both override
Enables DDNS updates of PTR RRs for all address pools except those configured
with the per-pool updatednscommand, which overrides global configuration. The
keywords are as follows:
both--(Optional) Enables the DHCP server to perform DDNS updates for A
and PTR RRs, unless the DHCP client has specified in the FQDN option that
the server should not perform the updates.
override--(Optional) Enables the DHCP server to perform DDNS updates for
PTR RRs even if the DHCP client has specified in the FQDN option that the
server should not perform the updates.
Note If you specify the bothand overridekeywords together, this enables the
DHCP server to perform DDNS updates for A and PTR RRs overriding
anything the DHCP client specified in the FQDN option to the contrary.
before--(Optional) Enables the DHCP server to perform DDNS updates before
sending the DHCP ACK back to the client. The default is to perform updates
after sending the DHCP ACK.
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Command or Action Purpose
Step 4 ipdhcp-clientupdatedns[server
{both| none}]
Example:
Router(config)# ip dhcp-client update dns server both
Enables DDNS updates of PTR RRs. The optional serverkeyword enables the
server to perform DDNS updates for A and PTR RRs. The keywords are as follows:
both--Enables the DHCP server to perform DDNS updates for A and PTRRRs, unless the DHCP client specifies in the FQDN option that the server
should not perform the updates.
none--Enables the DHCP client to perform DDNS updates and the server will
not perform any updates. The server can override this action.
Note The ipdhcp-clientupdatednsservernonecommand instructs the server not
to perform any updates. If configured to do so, the server can override the
client.
Note The ipdhcp-clientupdatednsserverbothcommand instructs the server to
update both the A and PTR RRs.
Step 5 exit
Example:
Router(config)# exit
Exits to privileged EXEC mode.
Examples
The following example shows how to configure A and PTR RR updates that are performed by the server
only:
ip dhcp-client update dns server both
ip dhcp update dns both override
Configuring DDNS Update Support on InterfacesPerform this task to configure your interfaces for DDNS update capability.
Note The interface configuration overrides the global configuration.
In order for DDNS updates to discover the DNS server, in cases in which the user did not configure the
server, the ipname-servercommand should be configured. This name server should be reachable by the
system, and the ipdomainlookupcommand should be configured (which is the default anyway). In cases
in which the configured hostname does not include a period (is not a fully qualified domain name[FQDN]), an IP domain name should be configured.
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Note The changes will not take effect until any current lease on the interface is released and a new lease is
requested that uses a new DHCP DISCOVER packet. This means configuring the ipaddressdhcp
command or using the releasedhcpEXEC command followed by the renewdhcpEXEC command.
>
SUMMARY STEPS
1. enable
2. configureterminal
3. interfaceinterface-typenumber
4. ipdhcpclientupdatedns[server{both| none}]
5. ipaddressdhcp
6. exit
DETAILED STEPS
Command or Action Purpose
Step 1 enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2 configureterminal
Example:
Router# configure terminal
Enters global configuration mode.
Step 3 interfaceinterface-typenumber
Example:
Router(config)# interface ethernet1
Specifies an interface type and number and enters interface configuration
mode.
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Command or Action Purpose
Step 4 ipdhcpclientupdatedns[server{both|
none}]
Example:
Router(config-if)# ip dhcp clientupdate dns server both
Configures the DHCP client to include an FQDN option when sending
packets to the DHCP server. The keywords are as follows:
both--(Optional) Enables the DHCP server to perform DDNS updatesfor A and PTR RRs, unless the DHCP client specifies in the FQDN
option that the server should not perform the updates.
none--(Optional) Enables the DHCP client to perform DDNS updates
and the server will not perform any updates. The server can override
this action.
Note The ipdhcpclientupdatednsservernonecommand instructs the
server not to perform any updates. If configured to do so, the server
can override the client.
Note The ipdhcpclientupdatednsserverbothcommand instructs the
server to update both the A and PTR RRs.
Step 5 ipaddressdhcp
Example:
Router(config-if)# ip address dhcp
Releases any current lease on the interface and enables the configuration.
Note You can also release any lease by using the releasedhcpEXEC
command followed by the renewdhcpEXEC command.
Step 6 exit
Example:
Router(config-if)# exit
Exits to privileged EXEC mode.
Configuring a Pool of DHCP Servers to Support DDNS UpdatesThere are two parts to the DDNS update configuration on the client side. First, if the ipddnsupdate
methodcommand is configured on the client, which specifies the DDNS-style updates, then the client will
be trying to generate or perform A updates. If the ipddnsupdatemethodddnsbothcommand is
configured, then the client will be trying to update both A and PTR RRs.
Second, the only way for the client to communicate with the server, with reference to what updates it is
generating or expecting the server to generate, is to include an FQDN option when communicating with the
server. Whether or not this option is included is controlled on the client side by the ipdhcp-clientupdate
dnscommand in global configuration mode or the ipdhcpclientupdatednscommand in interface
configuration mode.If the FQDN option is included in the DHCP interaction, then the client may instruct the server to update
reverse (the default), both, or none. Obviously, if the ipddnsupdatemethodcommand is
configured with the ddnsand bothkeywords, then the FQDN option configuration should reflect an IP
DHCP client update DNS server none, but you have to configure the system correctly.
Finally, even if the client instructs the server to update both or update none, the server can override the
client request and do whatever it was configured to do anyway. If there is an FQDN option in the DHCP
interaction as above, then server can communicate to the client that it was overridden, in which case the
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client will not perform the updates because it knows that the server has done the updates. Even if the server
is configured to perform the updates after sending the ACK (the default), it can still use the FQDN option
to instruct the client what updates it will be performing and thus the client will not do the same types of
updates.
If the server is configured with the updatednscommand with or without any keywords, and if the server
does not see an FQDN option in the DHCP interaction, then it will assume that the client does not
understand DDNS and will automatically act as though it were configured to update both A and PTR RRs
on behalf of the client.
Perform this task to configure a pool of DHCP servers to support DDNS updates.
In order for DDNS updates to discover the DNS server, in cases in which the user did not configure the
server, the ipname-servercommand should be configured. This name server should be reachable by the
system, and the ipdomainlookupcommand should be configured (which is the default anyway). In cases
in which the configured hostname does not include a period (is not a fully qualified domain name
[FQDN]), an IP domain name should be configured.
SUMMARY STEPS
1. enable
2. configureterminal
3. ipdhcppoolpool-name
4. updatedns[both| never] [override] [before]
5. exit
DETAILED STEPS
Command or Action Purpose
Step 1 enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2 configureterminal
Example:
Router# configure terminal
Enters global configuration mode.
Step 3 ipdhcppoolpool-name
Example:
Router(config)# ip dhcp pool test
Assigns a name to a DHCP pool and enters DHCP configuration mode.
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Command or Action Purpose
Step 4 updatedns[both| never] [override]
[before]
Example:
Router(dhcp-config)# update dnsnever
Enables DDNS update capability for a pool of DHCP servers for any addresses
assigned from this address pool.
If the server is configured using this command with or without any of the otherkeywords, and if the server does not see an FQDN option in the DHCP
interaction, then it will assume that the client does not understand DDNS and
act as though it were configured to update both A and PTR records on behalf
of the client.
The keywords are as follows:
both--(Optional) Perform forward and reverse updates. If the before
optional keyword is specified along with the bothkeyword, the server
can perform DDNS updates before sending the ACK back to the client.
If the overrideoptional keyword is specified with the bothkeyword, the
server can override the client and update forward and reverse RRs.
If the overrideand beforeoptional keywords are specified with the both
keyword, the server can override the client (forward and reverse updates) and
perform the updates before sending the ACK.
never--(Optional) Never perform updates for this pool.
override--(Optional) Override the client FQDN flags. If the before
optional keyword is specified, the updates will be performed before
sending the ACK.
before--(Optional) Perform updates before sending the ACK.
Step 5 exit
Example:
Router(dhcp-config)# exit
Exits to global configuration mode.
Examples
The following example shows how to configure a pool of DHCP servers to perform updates for A and PTR
RRs before the ACK is sent:
ip dhcp pool testupdate dns both before
Configuring the Update Method and IntervalPerform this task to specify the update method and interval maximum.
In order for DDNS updates to discover the DNS server, in cases in which the user did not configure the
server, the ipname-servercommand should be configured. This name server should be reachable by the
system, and the ipdomainlookupcommand should be configured (which is the default anyway). In cases
in which the configured hostname does not include a period (is not a fully qualified domain name
[FQDN]), an IP domain name should be configured.
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SUMMARY STEPS
1. enable
2. configureterminal
3. ipddnsupdatemethodmethod-name4. intervalminimumdayshoursminutesseconds
5. intervalmaximumdayshoursminutesseconds
6. ddns[both]
7. internal
8. http
9. addurl
10. removeurl
11. exit
12. exit
13. interfaceinterface-typenumber14. ipddnsupdatehosthamehostname
15. ip ddns update name
16. exit
DETAILED STEPS
Command or Action Purpose
Step 1 enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2 configureterminal
Example:
Router# configure terminal
Enters global configuration mode.
Step 3 ipddnsupdatemethodmethod-name
Example:
Router(config)# ip ddns update methodmyupdate
Specifies the update method name and enters DDNS update method
configuration mode.
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Command or Action Purpose
Step 4 intervalminimumdayshoursminutesseconds
Example:
Router(DDNS-update-method)# intervalminimum 1 0 0 0
Configures a minimum update interval. The arguments are as
follows:
days--Range is from 0 to 365. hours--Range is from 0 to 23.
minutes--Range is from 0 to 59.
seconds--Range is from 0 to 59.
Step 5 intervalmaximumdayshoursminutesseconds
Example:
Router(DDNS-update-method)# intervalmaximum 1 0 0 0
Configures a maximum update interval. The arguments are as
follows:
days--Range is from 0 to 365.
hours--Range is from 0 to 24.
minutes--Range is from 0 to 60.
seconds--Range is from 0 to 60.
Step 6 ddns[both]
Example:
Router(DDNS-update-method)# ddns
Configures DDNS as the update method. The bothkeyword specifies
that both A and PTR RRs will be updated.
Note You can specify DDNS or HTTP but not both in one step. If
you have specified DDNS, you must disable it by using the no
ddnscommand before you can configure HTTP. For the
HTTP configuration, see Steps 7,8, and 9.
Step 7 internal
Example:
Router(DDNS-update-method)# internal
Specifies that an internal cache will be used as the update method.
Step 8 http
Example:
Router(DDNS-update-method)# http
Configures HTTP as the update method and enters DDNS-HTTP
configuration mode.
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Command or Action Purpose
Step 9 addurl
Example:
Router(DDNS-HTTP)# add http://test:[email protected]/nic/update?system=dyndns&hostname=&myip=
Configures a URL that should be invoked in order to add or change a
mapping between a hostname and an IP address. The following
example configures the URL to be invoked to add or change the
mapping information using DynDNS.org:
http://userid:[email protected]/nic/update?
system=dyndns&hostname=&myip=.
You have to enter the URL string above. Userid is your userid and
password is your password at the DynDNS.org website. The special
character strings and will be substituted with the
hostname to update and the IP address with which that hostname
should be associated, respectively.
Note Before entering the question mark (?) character, press the
control (Ctrl) key and the v key together on your keyboard.
This will allow you to enter the ? without the software
interpreting the ? as a help query.
Step 10 removeurl
Example:
Router(DDNS-HTTP)# remove http://test:[email protected]/nic/update?system=dyndns&hostname=&myip=
Configures a URL that should be invoked in order to remove a
mapping between a hostname and an IP address. The URL takes the
same form as the addkeyword in Step 8.
Step 11 exit
Example:
Router(DDNS-HTTP)# exit
Exits to update-method configuration mode.
Step 12 exit
Example:
Router(DDNS-update-method)# exit
Exits to global configuration mode.
Step 13 interfaceinterface-typenumber
Example:
Router(config)# interface ether1
Enters interface configuration mode.
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Command or Action Purpose
Step 14 ipddnsupdatehosthamehostname
Example:
Router(config-if)# ip ddns updatehostname abc.dyndns.org
Specifies a host to be used for the updates. The update will associate
this hostname with the configured IP address of the interface. The
hostnameargument specifies the hostname that will receive the
updates (for example, DynDNS.org).
Step 15 ip ddns update name
Example:
Router(config-if) ip ddns update myupdate
Specifies the name of the update method to use for sending Dynamic
DNS updates associated with address changes on this interface.
Step 16 exit
Example:
Router(config)# exit
Exits to privileged EXEC mode.
Examples
The following example shows how to configure the update method, the maximum interval of the updates
(globally), and configure the hostname on the interface:
ip ddns update method mytestddnshttp!Before entering the question mark (?) character in the add http CLI, press the control
(Ctrl) key and the v key together on your keyboard. This will allow you to enter the ?without the software interpreting the ? as a help query.
add http://test:[email protected]/nic/update?system=dyndns&hostname=&myip=
interval maximum 1 0 0 0exitinterface ether1
ip ddns update hostname abc.dyndns.org
ip ddns update mytest
Verifying DDNS UpdatesUse the debugipddnsupdatecommand to verify that DDNS updates are being performed. There areseveral sample configurations and the debug output that would display for that scenario.
Sample Configuration #1
The following scenario has a client configured for IETF DDNS updating of A DNS RRs during which a
DHCP server is expected to update the PTR DNS RR. The DHCP client discovers the DNS server to
update using an SOA RR lookup since the IP address to the server to update is not specified. The DHCP
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client is configured to include an FQDN DHCP option and notifies the DHCP server that it will be updating
the A RRs.
!Configure the DHCP Clientip ddns update method testingddns
interface Ethernet1ip dhcp client update dnsip ddns update testingip address dhcpend!Configure the DHCP Serverip dhcp pool testnetwork 10.0.0.0 255.0.0.0update dns!Enable DebuggingRouter# debug ip ddns update00:14:39:%DHCP-6-ADDRESS_ASSIGN: Interface Ethernet1 assigned DHCP address 10.0.0.4, mask255.0.0.0, hostname canada_reserved00:14:39: DYNDNSUPD: Adding DNS mapping for canada_reserved.hacks 10.0.0.400:14:39: DYNDNSUPD: Sleeping for 3 seconds waiting for interface Ethernet1 configurationto settle00:14:42: DHCPC: Server performed PTR update00:14:42: DDNS: Enqueuing new DDNS update 'canada_reserved.hacks' 10.0.0.4
00:14:42: DDNS: Zone name for 'canada_reserved.hacks' is 'hacks'00:14:42: DDNS: Dynamic Update 1: (sending to server 10.19.192.32)00:14:42: DDNS: Zone = hacks00:14:42: DDNS: Prerequisite: canada_reserved.hacks not in use00:14:42: DDNS: Update: add canada_reserved.hacks IN A 10.0.0.400:14:42: DDNS: Dynamic DNS Update 1 (A) for host canada_reserved.hacks returned 0(NOERROR)00:14:42: DDNS: Update of 'canada_reserved.hacks' 10.0.0.4 finished00:14:42: DYNDNSUPD: Another update completed (total outstanding=0)
Sample Configuration #2
The following scenario has the client configured for IETF DDNS updating of both A and DNS RRs and
requesting that the DHCP server update neither. The DHCP client discovers the DNS server to update using
an SOA RR lookup since the IP address to the server to update is not specified. The DHCP client is
configured to include an FQDN DHCP option that instructs the DHCP server not to update either A or PTR
RRs. This is configured using the global version of the command.
!Configure the DHCP Clientip dh